1. Field of the Invention
The present invention relates to a backlight device and more particularly, to a direct type backlight device using Light-Emitting Diodes (LEDs) as its light source, and a Liquid Crystal Display (LCD) using the backlight device.
2. Description of the Related Art
As one use of LEDs, a white LED light source that emits white light is known, which is formed by combining a white (W) LED with three color LEDs of red (R), green (G) and blue (B) colors, or combining a white LED with some color LEDs other than red, green and blue LEDs. In recent years, the output power of LEDs has been increased at a high pace and thus, the applications of white LED light sources of this type have been becoming broader.
In particular, applications for illumination devices necessitating high brightness, light sources for projectors, and backlight devices for large-sized LCDs have been discussed. Since LEDs have the features that environmental load is small due to their mercury free property, color reproductivity is good, responsiveness is good, brightness is changeable, and lifetime is long, white LED light sources are expected as an alternative white light source to the conventional fluorescent lamps (i.e., hot cathode fluorescent lamps and cold cathode fluorescent lamps).
When white LED light sources are used for the aforementioned applications, i.e., illumination, light sources for projectors, and backlight devices for large-sized LCDs, a lot of LEDs each serving as a point light source is necessarily used in order to achieve required brightness to constitute an area light source in the present circumstances. Furthermore, brightness unevenness and chromaticity unevenness in the overall area light source need to be suppressed within predetermined ranges.
Here, when backlight devices using LEDs (i.e., LED backlight devices) are divided according to their structure, they may be classified into the edge light type and the direct type. These two types are typical.
With the edge light type, a light guide plate is disposed adjacent to a set of optical sheets including a diffusion sheet and a prism sheet, and illumination light emitted from aligned LEDs is inputted into one end face of the light guide plate. In this way, the illumination light from the LEDs is made perpendicular to the direction along which the illumination light is to be irradiated from the backlight device, thereby generating an area light source.
With the direct type, a diffusion plate is disposed adjacent to the lower surface of a set of optical sheets including a diffusion sheet and a prism sheet, and LEDs are arranged to have a predetermined pattern (which forms an LED array) directly below the diffusion plate. Illumination light emitted from the LEDs is inputted into one main surface of the diffusion plate. In this way, the illumination light from the LEDs is made parallel to the direction along which the illumination light is to be irradiated from the backlight device, thereby generating an area light source. It is said that the direct type is suitable for comparative large-sized backlight devices because this type has an advantage that brightness can be raised easily.
However, when the thickness of the direct type backlight device is limited, in other words, when the direct type backlight device has a thin profile, the optical path length from the LEDs to the diffusion plate is short and therefore, it is comparatively difficult to disperse uniformly the light from the respective color LEDs within the surface. As a result, there arises a difficulty that brightness unevenness and chromaticity unevenness are likely to occur. In particular, in the case of a white LED light source using three-color LEDs of red, green and blue colors, the arrangement positions of the LEDs are shifted from desired ones due to the contour size difference among the LEDs and at the same time, the emission peak of each color light is shifted from desired ones due to the arrangement position shifts of the LEDs. Accordingly, in-plane color unevenness is likely to occur.
The aforementioned color unevenness caused by the aforementioned arrangement position shifts of the three-color LEDs of red, green, and blue colors in the direct type backlight device with a thin profile does not occur in the central part (i.e., the part other than the peripheral part) of the same backlight device. This is because at any position in the central part, the light emitted from the LEDs is well mixed with the light from the surrounding LEDs to result in white light. The reason why color unevenness occurs in the peripheral part (i.e., in the neighborhood of the side wall) of the backlight device of this type is that the total amount of the light from the LEDs arranged along the side wall of the backlight device within the LED array (in other words, the LEDs arranged in the neighborhood of the side wall) is less than the total amount of the light from the LEDs arranged in the remaining part and thus, the light are not mixed to white light, resulting in a biased color of light which corresponds to the emission color of the LEDs arranged in the neighborhood of the same side wall.
As a method of improving the aforementioned color unevenness, Patent Literature 1 discloses a technique for enhancing color uniformity. In this technique, LEDs arranged in the neighborhood of a side wall are applied with a driving current different from that for LEDs arranged in the remaining positions. Thus, brightness of the LEDs in the neighborhood of the side wall is made lower than that of the LEDs in the remaining positions, thereby enhancing color uniformity.
The aforementioned technique disclosed in Patent Literature 1 is explained below with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view of the LCD device disclosed in Patent Literature 1 along a plane cutting the set of the LEDs. FIG. 2 is a drawing showing the technique that attenuates the brightness of the LEDs located at the ends of each strip in order to enhance color uniformity.
As shown in FIG. 1, this conventional LCD comprises a LCD layer 130 and a backlight device 132. The backlight device 132 comprises a lower mixing chamber 135, a diffusion film 101 as a first diffuser 176, an upper mixing chamber 175, a diffusion film 102 as a second diffuser 180, and a brightness enhancing film 182. The brightness enhancing film 182 is attached to the upper surface of an acrylic plate that constitutes the upper wall of the upper mixing chamber 175. The diffusion film 102 is attached to the lower surface of the same acrylic plate.
A predetermined number of three-color LEDs 134 of red, green, and blue colors are mounted on a circuit board 140 to have a predetermined layout in the lower mixing chamber 135. Each of the LEDs 134 comprises a side emission lens 142 and a spot reflector 166. The light emitted from the respective LEDs 134 is mixed in the lower mixing chamber 135 and diffused in the diffusion film 101 as the first diffuser 176, and thereafter, enters the upper mixing chamber 175 and mixed furthermore. Moreover, this light is diffused in the diffusion film 102 as the second diffuser 180 again, and propagates through the brightness enhancing film 182 placed on the upper surface of the backlight device 132 (the upper mixing chamber 175) to be irradiated to the LCD layer 130.
With the structure of FIG. 1, the color uniformity at the neighborhood (in the peripheral part) of the side wall 138 of the lower mixing chamber 135 of the backlight device 132 is enhanced by attenuating the optical outputs of some of the LEDs 134 that are located at the ends of the respective LED strips. The brightness of one to five LEDs 134 located at the ends of each LED strip can be attenuated according to the specific pattern or shape (e.g., the pitch and sequence of the LEDs 134) of the backlight device 132.
One of the various methods of attenuating the brightness of the LEDs 134 located at the ends of each LED strip is shown in FIG. 2. For simplification of explanation, a driver 1, a driver 2, and a driver 3 are illustrated to drive different groups of the LEDs 134 arranged along the LED strip in FIG. 2, respectively. For example, the driver 1 and 3 supply different electric currents from that supplied by the driver 2 to the corresponding red, green and blue LEDs 134 arranged along the LED strip in order to achieve desired color balance of red, green and blue colors with respect to the designated white point.
In FIG. 2, the last three LEDs 134 arranged at each end part of the LED strip are respectively driven by lower driving currents than that for the color LEDs 134 arranged at the remaining positions in the same LED strip. For example, the brightness of the green LEDs 134 located at the left and right end positions of the LED strip is set to be approximately half of the brightness of the green LEDs 134 located in the middle part of the LED strip. The brightness of the LEDs 134 located inwardly next to the left and right end positions of the LED strip is set to be approximately 50% to 75% of the brightness of the same color LEDs 134 located in the middle part of the LED strip. The brightness of the LEDs 134 located inwardly next but one to the left and right end positions of the LED strip is set to be approximately 60% to 90% of the brightness of the same color LEDs 134 located in the middle part of the LED strip. In this way, the brightness of the LEDs 134 arranged at the left and right end positions of the LED strip is reduced compared with the brightness of the same color LEDs 134 arranged at the middle part of the LED strip. In addition, the specific attenuation quantity of the brightness level is based on human perception for color uniformity, and the optimum attenuation quantity can be judged experimentally.
However, driving the LEDs 134 located in the end parts of the LED strip and the LEDs 134 located in the middle part thereof by different drivers similar to the aforementioned conventional backlight device 132 will induce problems, such as increase in costs due to increase of the drivers and their control circuits, and occupation space increase (this leads to contour change of the LCD).